Primates express dominant restriction factors that block the infection of certain retroviruses soon after the virus enters the host cell but prior to reverse transcription. The retroviral capsid is the determinant of susceptibility to restriction. TRIM5a or, in some species, TRIMCyp mediates the earl blocks to retroviral infection in primate cells. TRIM5a is a member of the tripartite motif (TRIM) family of proteins and contains RING, B-box 2 and coiled-coil domains. TRIM5a also contains a carboxy-terminal B30.2/SPRY domain, which has been shown to contribute to capsid recognition. Differences among the B30.2/SPRY domains of TRIM5a proteins from different primate species account for the observed patterns of retroviral restriction. For example, human immunodeficiency virus (HIV-1), the cause of acquired immunodeficiency syndrome (AIDS), is potently restricted by the TRIM5a protein from Old World monkeys, but only modestly inhibited by human TRIM5a. A single arginine residue within a variable region of the human TRIM5a B30.2 domain decreases the affinity for the HIV-1 capsid, resulting in a diminution of restriction. An understanding of the structural basis of capsid recognition by TRIM5a may suggestion approaches to potentiate this innate intracellular immunity to retroviruses. The overall goal of the work proposed is to understand the structure of primate TRIM5a and TRIMCyp proteins and to investigate their interaction with the assembled retroviral capsid. The specific aims of this proposed are: 1) To establish and optimize systems for the expression of primate TRIM5a and TRIMCyp proteins; 2) To create TRIM5a and TRIMCyp fragments that are suitable for structural analysis; 3) To identify and characterize ligands for TRIM5a that could promote crystallization. Human immunodeficiency virus (HIV) cannot infect monkeys because these animals make a resistance factor called TRIM5alpha. Human TRIM5alpha only modestly blocks HIV. The proposed work will attempt to obtain a detailed picture of TRIM5alpha so that its anti-HIV activity could be improved. [unreadable] [unreadable] [unreadable]